1. Field of the Invention
The present invention relates to an image forming apparatus and method that employ thermal development type, for forming an image on a photothermographic imaging material by thermal development.
2. Description of Related Art
A laser imager used in medical field is known as an example of an image forming apparatus. The laser imager is an apparatus for visualizing a latent image by performing development after exposing laser on a film as a phototmermographic imaging material, on a surface of which a photosensitive agent is formed. The laser imager is capable of outputting an image of high resolution and high gradation compared to the display by a display, such as CRT, or the printout to a paper by a copying machine. Therefore, it is well used in the medical field. For example, a laser imager is used for visualizing output data of an X-ray CT apparatus (X-ray computed tomography apparatus), an NMR (nuclear magnetic resonance apparatus), an ultrasonic wave CT apparatus (ultrasonic wave computed tomography) or the like.
After a laser beam is modulated by a light modulation element, it is irradiated on the film by being scanned. The exposed film is developed by heat or developing solution. Thereby, an image is visualized on the film. Since the light modulation element modulates the laser beam in accordance with the output data of the X-ray CT apparatus or the like, the output data is visualized on the film.
There are wet type and dry type in development types. However, the dry type has been increasing because of its easiness in handling. In case of using the dry type, thermal development type for heating the exposed film by a thermal development drum is employed in many cases.
In thermal development, the development amount (finished density) is determined by how much time the film is kept at not less than a critical temperature at which thermal development reaction is caused. Therefore, in the image forming apparatus for performing thermal development, the heating temperature or the development time at the time of development is an extremely important factor in order to maintain the finishing of the photothermographic imaging material well. In particular, in the image forming apparatus of high gradation and high resolution, such as a laser imager or the like, the effect of the variation in development amount on the finishing of the film is comparatively large.
The apparatus in Japanese Patent Laid-Open Publication No. 2000-347311 is an example of an image forming apparatus that employs thermal development type. The image forming apparatus comprises a feeding portion for feeding films as sheet-like photothermographic imaging materials one by one, an exposure portion for exposing the fed film, and a thermal development portion for thermally developing and cooling the exposed film.
The exposure portion is provided at the lower portion in the housing of the image forming apparatus. The exposure portion irradiates a laser beam in a range of 780 to 860 nm of infrared region on the photosensitive surface of the film in a horizontal direction so as to scan and expose the film, so that a latent image in accordance with the exposing image signal is formed.
Further, the thermal development portion having a heating portion and a cooling and conveying portion is provided on the upper portion of the image forming apparatus. The heating portion comprises a thermal development drum having a built-in heat source, and facing rollers for conveying the film on the thermal development drum. In the heating portion, the thermal development drum heats and thermally develops the film while the film and the outer circumferential surface of the thermal development drum are rotated to the conveying direction together in a state of being in close contact with each other. Thereby, the latent image of the film is formed to be a visible image.
In the latter step of the heating portion, a plurality of conveying roller pairs are provided, and the cooling and conveying portion for conveying and cooling the film separated from the thermal development drum is provided. Then, the conveying roller pairs convey the cooled film in the cooling and conveying portion, and the film is outputted to a film outputting portion provided, for example, on the upper portion of the image forming apparatus.
However, in the image forming apparatus in earlier technology, it is required to control the heating temperature of the thermal development drum in the heating portion uniformly in order to make the image density on the film after developed uniform. However, it is insufficient just to control the heating temperature of the thermal development drum. There are some problems as follows.
That is, in such an image forming apparatus, when the film is thermally developed continuously, temperature variation in the thermal development portion of the image forming apparatus, such as temperature decrease of the facing rollers according to the heat being taken away by the film, temperature increase of the cooling and conveying portion according to the heat supply from the heated film, or the like, is caused.
According to such an effect of temperature variation in the thermal development portion, temperature difference in processing is caused for every film processed continuously. As a result, predetermined density characteristics cannot be obtained in the film after developed, so that density difference in development is caused.
As a measure for this problem, an image forming apparatus for performing development of a predetermined density, comprising a feedback mechanism for adjusting the development density by measuring the density of the film after developed and performing adjustment of the intensity of exposure in the exposure portion or adjustment of the heating amount in the thermal development portion according to the measured density.
However, the cost of such an image forming apparatus comprising a feedback mechanism for adjusting the development density on the basis of measured density increases since the members constituting the apparatus increase. Moreover, there is a problem that the apparatus itself becomes complicated.
Further, the thermal development drum in the heating portion is the heat source for heating directly the film, and is the portion which affects the image density after development most. However, at the time of exposure in the exposure portion, the temperatures in the inside and in the vicinity of the exposure portion vary because of the installation environment including winter, summer, air conditioning or the like. Thereby, the wavelength of the semiconductor laser having temperature dependency varies, so that the intensity of beam over the photosensitive wavelength region that the film receives varies substantially. Therefore, already different density of latent image becomes to be formed. Accordingly, the finished density also varies even though the thermal development portion is in constant condition. Thus, the effect of the temperature of the exposure portion also increases.